1. Field of the Invention
The present invention relates to a method for manufacturing a hydro dynamic bearing device. In particular, the present invention relates to a method for manufacturing a hydro dynamic bearing device to be used in a spindle motor equipped in an information technology device such as a magnetic disk device (e.g., HDD or FDD), an optical disk device (e.g., CD-ROM, CD-R/RW, or DVD-ROM/RAM), and an optical magnetic disk device (e.g., MD or MO), a scanner motor equipped in a copying machine, a laser printer (LBP), a barcode reader, or the like, or a small-sized motor equipped in an electrical equipment such as an axial fan.
2. Description of the Related Art
As is generally known in the art, each kind of the motors listed above have been promoted to be provided at lower cost, driven at higher speed and more quiet, and so on in addition to attain a high rotational accuracy. As one of factors that define these required performances, a bearing supporting spindle of the motor has been increasingly valued. In recent years, therefore, as such a kind of the bearing, the use of a hydro dynamic bearing device having excellent characteristics that serve a request for the above performance has been studied, and such a hydro dynamic bearing device has been developed in a quest to put it to practical use.
For instance, a hydro dynamic bearing device to be built in a spindle motor of a disk device such as a hard disk drive (HDD) comprises a radial bearing part for rotatably retaining an axial member in a non-contact manner in the radial direction and a thrust bearing part rotatably retaining the axial member in a non-contact manner in the thrust direction. As a bearing part of each of them, a hydro dynamic bearing device having a groove (a hydro dynamic pressure generating groove) for the generation of hydro dynamic pressure on its bearing surface is used.
In this case, the hydro dynamic pressure generating groove of the radial bearing part is formed in the inner peripheral surface of the bearing member or the housing, or formed in the outer peripheral surface of the axial member. On the other hand, in the case of using an axial member having a flange part, the hydro dynamic pressure generating groove of the thrust bearing part is formed in each of the opposite end faces of the flange part or the surface (e.g., the end face of the bearing member or the bottom surface of the housing) facing to such an end face.
In such a kind of the spindle motor, furthermore, a higher rotational accuracy has been desired in recent years for attaining an increase in information-recording density and an increase in rotation speed of the motor. For addressing such a request, a higher rotational accuracy of the hydro dynamic bearing device to be built in the spindle motor has also been desired.
To improve the rotational accuracy of the hydro dynamic bearing device, it is important to adjust the radial bearing clearance and the thrust bearing clearance in which hydro dynamic pressures are generated. For properly adjusting the clearance, there is a need to work upon the structural component of the hydro dynamic bearing device related to each bearing clearance, especially the axial member that forms each axial bearing with the axial member. Therefore, the required grinding is performed as a finish machining on the part where each bearing clearance of the axial member is formed at the time of processing or manufacturing the axial member.
More concretely, as shown in FIG. 10, the axial member 2 is comprised of an axial part 2a and a flange part 2b, which are molded in one piece. A bearing member (not shown) is arranged on the outer peripheral side of the axial member 2. In addition, a radial bearing clearance is formed between the outer peripheral surface of the axial part 2a and the bearing member. Furthermore, a thrust bearing clearance is formed between the distal end face 2b1 of the flange part 2b (the end face on the near side to the axial part 2a) and the bearing member, and also another thrust bearing clearance is formed between the proximal end face 2b2 of the flange part 2b (the end Face on the far side from the axial part 2a) and the inner bottom face of the housing (not shown).
In this case, in the process of manufacturing the axial member 2, the outer peripheral surface of the axial part 2a, which forms the radial bearing clearance with the bearing member, is subjected to grinding. For grinding the outer peripheral surface, heretofore, the following exemplified method has been generally applied.
As shown in FIG. 11, the conventional method comprises the steps of perforating the center of the distal end face 2a4 of the axial part 2a formed on the axial member 2 and the center of the proximal end face 2b2 of the flange part in the axial member with center holes 2bc, respectively, and fitting a pair of tapered centering members 41 into the center holes 2bc, respectively, to sandwich the axial member 2 with the centering members 41.
Under such a condition, the grinding is performed by imparting an axial rotary motion from the centering member 41 to the axial member 2, while pressing a grindstone 43 on the outer peripheral surface.
However, when the grinding is performed while supporting the axial member from its opposite ends with the respective centering members 41, there is a possibility of causing a decrease in not only a grinding efficiency but also a work efficiency because of the following reasons. That is, the circumferential speed of the axial member 2 is hardly increased in a sufficient manner at the time of the rotary motion of the axial member 2 (e.g., it is limited to about 100 rpm) due to the facts, for example the contact area between the centering member 41 and the center hole 2bc is small.
In the grinding with such a center support, the axial member may cause centrifugal whirling while rotating when the center position is deviated. As a result, the quality of the axial member may decrease as the outer peripheral surface of the axial part of the axial member may be ground in a slanting direction, or the roundness of the axial member may deviate from the desired level.
The present invention has been completed in consideration of the above circumstances. A technical object of the present invention is to provide a method for manufacturing a hydro dynamic bearing device capable of sufficiently increasing a circumferential speed of an axial member in the step of grinding the axial member and capable of preventing the generation of centrifugal whirling of the axial member to increase the grinding efficiency and the working efficiency in addition to improve the quality of the resulting product.
In the present invention, for attaining the above technical object, there is provided a method for manufacturing a hydro dynamic bearing device including an axial member having a flange part on one end of an axial part thereof a radial bearing part that support the axial part in a non-contact manner in a radial direction by a hydro dynamic pressure action of a fluid generated in a radial bearing clearance, and a thrust bearing part that supports the flange part in a non-contact manner in a thrust direction by a hydro dynamic pressure action of a fluid generated in a thrust bearing clearance. The method includes, the steps of: supporting the axial member at both ends of the axial member in an axial direction thereof with a pair of plate members in a face-contact manner, rotating the axial member around its axial center, and grinding an outer peripheral surface of the axial part of the axial member on a grindstone while supporting the outer peripheral surface of the axial part with a supporting member.
Here, the phrase xe2x80x9csupporting the axial member at both ends of the axial member with a pair of plate members in a face-contact mannerxe2x80x9d means that the proximal end face of the flange part of the axial member (i.e., the end face of the flange part on the far side from the axial part) is supported by one of the plate members such as a backing plate in a face-contact manner and the distal end face of the axial part of the axial member is supported by the other of the plate members such as a pressure plate in a face-contact manner to sandwich the axial member between the plate members with their appropriate holding powers.
According to such a configuration of the method, the opposite end faces of the axial member is supported by a pair of the plate member in a face-contact manner, so that the contact area between each end face and the corresponding plate member becomes wide compared with that of the conventional center-supporting method and the slip between the end face and the corresponding plate member hardly occurs, resulting in a preferable contact state for obtaining a larger holding power. Therefore, in the case of imparting the axial member a rotary motion around the axle center thereof, the circumferential speed of the axial member can be sufficiently increased, resulting in the improvements of grinding efficiency and working efficiency. Furthermore, the opposite ends of the axial member are supported in a face-contact manner instead of being supported by means of the center support as that of the conventional one. Therefore, the generation of centrifugal whirling hardly occurs at the time of the rotation of the axial member, so that it will be advantageous for grinding the outer peripheral surface of the axial part with high precision. Here, the supporting member for supporting the outer peripheral surface of the axial part of the axial member may be placed almost on the middle of the axial member in the axial direction. In this case, it is advantageous to perform grinding in the form of pressing a grindstone on the whole length of the outer peripheral surface of the axial part in the axial direction. In addition, the number of the supporting members is not specifically limited. Preferably, however, one or two supporting members may be placed in their respective positions facing to the grindstone (i.e., the positions that receive the pressures from the grindstone).
In this case, the plate member to be face contact with the flange part of the axial member (i.e., the end face of the flange part on the far side from the axial part) may preferably includes a roll-off part on a predetermined area defined in a rotational center of the contact surface. That is, when the step of cutting the end face of the axial member is performed prior to the step of grinding, the circumferential speed of the outer peripheral part of the end face of the axial member is relatively high at the time of a rotary motion of the axial member. Therefore, an appropriate cutting can be performed using a cutting tool. In this case, on the other hand, the circumferential speed of the rotational center of the end face is relatively low, so that a sufficient cutting cannot be performed. After completing the cutting, there is caused an undesired protruded part in the vicinity of the rotational center of the end face of the axial member (particularly, the end face of the flange part). In order to avoid such a disadvantage, as described above, at least a predetermined area defined in the rotational center of the plate member to be face contact with the end face of the flange part may be provided with a roll-off part to prevent the protruded part formed on the end face from contacting with the plate member at the time of cutting, securing an appropriate state of face contact.
It is preferable that the outer diameter d of the contact surface of the plate member to be face contact with the flange part is defined so as to be smaller than the outer diameter D of the flange part. Such a configuration allows the contact surface to be hardly slipped while the difference with a supporting state of the end face on the axial part side because of an increase in the holding pressure per unit area acting on the contact surface, compared with the case in which the contact surface of the plate member is set to an almost similar size to the flange diameter of the contact surface of the plate member. Consequently, the axial member can be supported from its opposite ends with a good balance and a good stability.
In the above configuration of the device, preferably, the contacting part of the above plate member to the flange part may be elastically supported by an elastic member. In this case, the contacting part of the plate member to the flange part may be preferably comprised of, for example, a metallic plate or the like having a high rigidity. In this way, an appropriate elastic deformation of the elastic member allows the contacting part of the plate member to be brought into contact with the flange part all over the contacting part without any clearance between them in a face-contact manner. As the state of face-contacting support to the flange part is stabilized, it becomes possible to grind the outer peripheral surface of the axial part with high precision.
In addition, the supporting member may preferably support two thirds or more of the outer peripheral surface of the axial member in the direction of the axle center thereof Therefore, the press force from a grindstone is uniformly received by the supporting member over a wide range in the direction of the axle center, so that it becomes possible to grind the outer peripheral surface with high precision without letting the axial part of the axial member rattle or oscillate.
In this case, as described above, in addition to the step of grinding the outer peripheral surface, when the additional step of grinding one end face of the flange part on the axial part side and the other end face of the flange part on the far side from the axial part, the end face on the far side from the axial part may preferably be ground in advance of grinding the end face on the axial part side. Consequently, the end face of the flange pan on the far side from the axial part is ground with high precision at first, and then the finished end face is supported by the supporting member while grinding the other end face of the flange part on the axial part side, so that both end faces of the flange part can be ground with high precision. On the other hand, when the end face of the flange part on the axial part side is ground at first, the end face of the flange part on the axial part side cannot be effectively used even though such an end surface has finished with high precision by the grinding because the finished end face is not involved in the step of grinding the end face of the flange part on the far side from the axial part. Consequently, as described above, when the end face of the flange part on the far side from the axial part is ground first the ground end face can be effectively used, resulting an appropriate sequence of grinding operations.